As heightening in integration and densification of semiconductor devices has been furthered, a lithographic patterning on the level of 45 nm is now being realized. Such pattering can be effectively achieved by improved technologies such as immersion exposure method and double exposure method, which use ArF instead of conventional excimer light. However, to cope with a next-generation patterning on the level of 32 nm or even thinner, the exposure technology depending on excimer light falls short, and an EUV exposure technology, which uses EUV light of which the main wavelength is 13.5 nm, far shorter than that of the excimer light, is hoped to be the solution.
Although quite an improvement has been made toward realization of this EUV exposure technology, there remain many technical problems to be solved with respect to light source, resist, pellicle, etc. For example, with regard to the pellicle used to prevent adhesion of a foreign matter to a photomask, which is a phenomenon that lowers production yield, there are various unsolved problems, and thus the pellicle poses a big obstacle against the realization of EUV exposure technology. An especially difficult problem lies in that there has not been a clear roadmap toward realizing a development of a material to make the transparent film of the pellicle that does not age with oxidation or the like and is thus chemically stable, in addition to having a high transmittance of EUV light.
To pass as a good material to make such a pellicle film for EUV, conventional materials have diverse problems, and in particular organic substances do not effectively pass an EUV light and are decomposed and degraded by the EUV light. Although there exists no material that has a perfect transparency to the wavelength range of EUV light, there are disclosed silicon thin films as relatively transparent films for EUV light (ref. IP Publication 1 and non-IP Publication 1).
These silicon films for EUV application are desired to be as thin as possible in view of reducing the attenuation of EUV light; however, these silicon thin films, which are made up of a silicon membrane of 20 nm thickness and a rubidium layer of 15 nm thickness, have thicknesses on nanometer order, so that they are physically very fragile and cannot stand on its own as a pellicle film for EUV.
For this reason, in order to solve this difficulty inherent to using such silicon thin film as a pellicle film for EUV, there has been proposed a use of a structure which has a honeycomb-like shape and has openings adapted to pass EUV light and which is adhered to the extremely thin silicon film to reinforce it. For example, there is proposed a pellicle for EUV which utilizes an SOI (silicon on insulator), and this pellicle has a meshed honeycomb-like structure for reinforcement of the pellicle membrane for EUV (IP Publication 2).
As a mesh structure for reinforcement of the pellicle film for EUV, one can choose from various configurations besides the above-mentioned honeycomb-like structure so long as the intended purpose is met, such as a square or rectangular lattice structure, a plate body having openings in an arbitrary shape such as circle and polygon. The strength of the structure is determined by mesh pitch, width of the thin grid frame defining each mesh opening, and height (thickness) of the frame; and the strength of the structure is greater with narrower pitch, greater width of the grid frame and greater height of the grid frame.
Since this mesh structure does not pass EUV light except through its openings, the opening ratio of the mesh structure ought to be increased in order that the attenuation of the incoming EUV light is minimized. However, the strength of the pellicle film for EUV and the opening ratio of the mesh structure are in a trade off relationship, so that the higher the strength of the pellicle film is, the lower the opening ratio and thus the transmittance become, and as a result, the opening ratio of the mesh structure has to be compromised in designing to meet the operation conditions and restrictions.
The pellicle for EUV as reinforced by such mesh structure is used in an EUV exposure machine as shown in FIG. 10 so as to isolate the pattern area of the mask from the dust and particles. Incidentally, a typical mask is rectangular having straight side lines. In this EUV exposure machine, the EUV light that is emitted from the light source 1 in the stepper enters and passes through the pellicle 2 with an incidence angle of 4-8 degrees as an illumination light and reaches the mask 3. The EUV light that has reached the mask 3 is reflected by the mask 3 and passes the pellicle 2 for the second time, and then enters the optical system 4 and forms an image on the wafer 5.
Now, the EUV light shot from the light source 1 passes through the pellicle 2 twice, as it goes to the mask 3 and reflects therefrom, before it transfers the pattern on the mask 3 to the wafer 5. On this occasion, if the pellicle 2 is reinforced by the mesh structure, as described above, a part of the EUV light is intercepted by the mesh structure, which is impermeable to the EUV light, with a result that the image of the mesh structure is cast on the wafer 5 and this would adversely affects the exposing operation.